Pb is usually added to a Cu—Sn based copper-alloy for sliding use. Pb has a melting point of 327 degrees C. Pb on the sliding surface expands and is elongated upon temperature rise during the sliding. Because Pb cools the sliding surface and exhibits its excellent self-lubricating properties, seizure is prevented. In addition, since Pb forms a soft dispersing phase, it provides conformability and allows foreign matters to the embedded therein.
However, Pb of the Cu—Pb based alloy is harmful to the human body and environment. To address such concerns, there have been proposed Pb-free alloys which attain the sliding performances equivalent to those of Cu—Pb based alloys. Bi is metal as soft as Pb and Cu—Bi based alloy forms a liquid phase under sliding conditions. In term of hardness, however, the effect of Bi to enhance sliding performances seems to be inferior to that of Pb.
Patent Document 1 (Japanese Unexamined Patent Publication (kokai) No. 10-330868) consists of from 5 to 50 mass % of Bi phase, and balance of Cu phase and inevitable impurities. The Bi phase consists of Bi or a Bi based alloy. The Bi phase contains at least one element of Sn, Ag, or In. Ranges of such at least one element contained in the Bi phase are 20% by weight or less of Sn, 10% by weight or less of Ag, and 5% by weight or less of In. The sintering conditions employed in the Examples of Patent Document 1 are, with regard to the starting materials, a combination of pure Cu and pure Bi, a combination of pure Cu and Bi alloy (such as a Bi—In based alloy), a combination of phosphor bronze and Bi alloy (such as Bi—In alloy), and with regard to the thermal condition 800 degrees C. for 1 hour. As a result of diffusion between pure copper (or Cu-based alloy powder) and pure Bi (or Bi-based alloy mentioned above) in a sintered copper alloy, triple joints and their the vicinity of grain boundaries of the Cu-alloy matrix are places, where a Bi phase is present, as shown in Patent Document 1. In added to the Bi alloy with addition of Bi is alloyed with the Bi phase and a resultant Bi—In phase forms a grain-boundary phase.
A composition of the sintered copper alloy proposed in Patent Document 2 (Japanese Patent No. 3421724) is free In. The alloy is composed of, from 5 to 15% by weight of Sn, from 1 to 20% by weight of Bi, from 0.1 to 10% by volume of hard particles, the balance being Cu. The hard particles co-present in the Bi phase prevent Pb and Bi from flowing. Since the Pb or Bi phase acts as a cushion of hard particles, they attack an opposite shaft in only reduced amounts. Separated hard particles are again captured by the Pb or Bi phase, and therefore, abrasive wear is less likely to occur.
In Patent Document 2, powder mixture consisting of Bi powder, powder of hard particles, Sn powder, and Cu powder is sintered and rolled to finish sliding material. Sn strengthens the matrix of finished sliding material. Hard particles are enveloped in the Bi phase. It is considered that: during sintering process Cu—Sn mutually diffuse while separating Bi to form a Bi phase and moving Bi moves to the location of the hard particles. Referring to FIG. 1 of this Patent Document, grain boundaries and the grain-boundary triple joints of Cu alloy particles are the place where the Bi phase is present.
The Cu—Bi (Pb) based sintered alloy according to Patent Document 3 (Japanese Unexamined Patent Publication (kokai) 2001-220630) is free of In and discloses as follows. An intermetallic compound is added to enhance wear resistance. The structure is such that the intermetallic compound is present at the circumference of Bi or Pb phase. During sliding an intermetallic compound protrudes higher than surface of the copper alloy, while the Bi and Pb phases and the Cu matrix become so concave that oil is reserved therein. The so-provided sliding material has improved seizure resistance and fatigue resistance according to the disclosure of the patent document. Exemplary the sintering conditions mentioned are 800-920 degrees C. for approximately 15 minutes.
Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. 10-330868
Patent Document 2: Japanese Patent No. 3421724
Patent Document 3: Japanese Unexamined Patent Publication (kokai) No. 2001-22063